Determination of effective energies in CT calibration

Abstract

The effective energy of a polychromatic beam for a Computed Tomography (CT) scanner can be measured directly only with difficulty. However, a linear relationship exists between the measured CT numbers and corresponding attenuation coefficients of known materials at the effective energy of the x-ray beam. The effective energy can then be determined by searching all energies for the best linear correlation between the CT numbers and the attenuation coefficients. This can be performed by two methods: graphically, by means of choosing visually the straightest of the fitted lines or, mathematically, by maximizing the correlation coefficient. The energy corresponding to the optimal fit is therefore selected as the effective energy. The latter method was implemented by computer and demonstrated by scanning the AAPM phantom, which contained known materials, and determining the effective energies and the relationship between the linear attenuation coefficients and CT numbers for three commercial units.

Mapping of foam mobility in porous media

“…PEEK is also believed to transmit X-rays within a narrow energy window (refiltering the polyenergetic source X-rays), which appreciably minimizes the beam-hardening artifact due to the polychromaticity (sometimes called the interpetrous lucency artifact (Brooks et al, 1978)). This effect gives the appearance of dark streaks and flares in the vicinity of the boundary between the coreholder and the core (note that streaks can also be caused by aliasing (Millner et al, 1978)). In all experiments, the coreholder is placed vertically on the platform of the CT scanner apparatus and was kept in place using a polymethyl-methacrylate (PMMA) stand.…”

“…These effects are customarily evaluated together through a single coefficient, μ , called the attenuation coefficient of the material (Kak, 1979;. This constant is also dependent on the X-ray energy (Brooks et al, 1978;Millner et al, 1978). In commercial scanners, the numbers actually put out by the computer attached to the scanner are integers, called the CT values.…”

“…The X-ray tube of the CT-Scanner is operated at either 130 kV or 110 kV. The corresponding effective energies, at which a given material will exhibit the same attenuation coefficient as measured by the scanner, are typically about 50 to 60% of these values (see Millner et al, 1978) for a practical procedure for determining the effective energy of a CT scanner). The use of low X-ray energy gives rise to the undesirable X-ray scattering effect.…”

Statistical characterization of noise for spatial standardization of CT scans: Enabling comparison with multiple kernels and doses

“…Although CT scanners are calibrated as part of the imaging workflow, the calibration is circumscribed to global reference values of air and water (Millner et al, 1978). This fact jointly to other inherent factors of the acquisition makes the CT signal more variable than desired (e.g.…”

Gas Trapping During Foamed Flow in Porous Media

“…PEEK is also believed to transmit X-rays within a narrow energy window (refiltering the polyenergetic source X-rays), which appreciably minimizes the beamhardening artifact due to the polychromaticity (sometimes called the interpetrous lucency artifact [Brooks et al, 1998]). This effect gives the appearance of dark streaks and flares in the vicinity of the boundary between the coreholder and the core (note that streaks can also be caused by aliasing [Millner et al, 1978]). The geometry and structure of the coreholder are also designed to minimize beam hardening and scattering artifacts in longitudinal imaging where the detected energy spectrum may vary with the projection direction.…”